An internal combustion engine operating under various conditions can experience a build-up of combustion chamber deposits (‘CCD’), which comprise a deposition of products of the combustion process onto surfaces of the combustion chamber. The deposits are typically derived from fuels and fuel additives, lubricating oils and oil additives, and other elements, as is known.
As deposits adhere to surfaces of the combustion chamber, thermal characteristics of the engine change. This is primarily due to the combustion chamber deposits acting as an insulating layer on the combustion chamber surfaces, which affects combustion. The result of the formation of the insulating layer includes a decrease in maximum and average heat flux away from the combustion chamber, a decrease in heat transfer to engine coolant, and a decrease in engine breathability, i.e. air flow, leading to a reduced volumetric efficiency. Furthermore, the resultant effects include reduced engine power, a potential for increase in NOx emissions, and an increased likelihood of pre-ignition, or knock. There may be a benefit of improved fuel economy and reduced CO2 emissions.
It is known that to improve thermal efficiency of gasoline internal combustion engines, dilute combustion—using either air or re-circulated exhaust gas—gives enhanced thermal efficiency and low NOx emissions. However, there is a limit at which an engine can be operated with a diluted mixture because of misfire and combustion instability as a result of a slow burn. Known methods to extend the dilution limit include operating the engine under controlled auto-ignition combustion.
One engine system being developed for controlled auto-ignition combustion operation comprises an internal combustion engine designed to operate under an Otto cycle. The engine is preferably equipped with direct in-cylinder fuel-injection and a spark ignition system to supplement the auto-ignition process under limited operating conditions. Such engines are referred to as Homogeneous Charge Compression Ignition, or HCCI engines.
In the HCCI engine, a charge mixture of combusted gases, air, and fuel is created in a combustion chamber, and auto-ignition is initiated simultaneously from many ignition sites within the charge mixture during a compression stroke, resulting in stable power output and high thermal efficiency. Since combustion is highly diluted and uniformly distributed throughout the charge mixture, the burnt gas temperature and hence NOx emissions are typically substantially lower than NOx emissions of a traditional spark ignition engine, and of a traditional diesel engine.
A typical HCCI engine is distinguishable from a spark-ignition engine in that ignition of the charge mixture is caused by compression of the charge mixture. A typical HCCI engine is distinguishable from a compression-ignition engine in that the compression-ignition engine initiates ignition of the combustion charge by injection of fuel, whereas the fuel charge for the typical HCCI engine is preferably injected into the combustion chamber at a time prior to start of ignition of the charge mixture.
It is known that combustion chamber deposits (CCD) form a thermal insulation layer within the cylinder. Combustion chamber deposits have been shown to extend the operating range of HCCI combustion. However, CCD properties and thickness are subject to change during engine operation. Due to different operating characteristics, when combustion chamber deposits are formed on surfaces of a combustion chamber for the typical HCCI engine, there is a resultant change in timing of auto-ignition of the charge mixture and a change in rate of heat release during combustion. This leads to varying combustion performance over time, as shown with reference now to FIGS. 4A and 4B. FIG. 4A comprises a graph of evolving cycle heat release rate at difference instances of a 40-hour test in which an exemplary HCCI engine was operated at 2000 RPM, at an air/fuel ratio of 20:1, injection of 11 milligrams (mg) fuel/cycle, in order to track changes in combustion due to gradual formation of in-cylinder combustion chamber deposits. FIG. 4B comprises a scatter plot showing 100 cycles of a sweep of operating points showing the effect of CCD on HCCI operation. The scatter of cycles shows 10-90% burn duration versus CA10 after different amounts of operation time. By way of explanation, the ignition timing of controlled auto-ignition combustion is defined as the crank angle position at which 10% of the mass fraction of the combustion chamber charge is burned, also referred to as CA10. The burn duration of combustion is defined as the crank angle interval between 10 and 90% mass fraction burned. The results shown with reference to FIGS. 4A and 4B demonstrate that there is a significant change in timing of charge ignition and rate of charge combustion over time of operation attributable to changes in thermal characteristics of the combustion chamber surface. It is demonstrated that magnitude of CCDs has a significant effect on the performance of controlled auto-ignition combustion engines. The effect is that the level of CCD formation needs to be considered in calibration and control of HCCI engine operation.
In the HCCI engine with multiple cylinders, combustion timing for each cylinder can vary significantly due to differences in intake conditions and thermal boundary conditions of individual cylinders, which is further exacerbated by presence and buildup of combustion chamber deposits.
Throttle and EGR valve positions can influence combustion timing but the effects are global, i.e. affecting all cylinders essentially equally. Combustion phasing can be controlled by varying intake/exhaust valve lift profiles and timings for individual cylinders, when an engine is so equipped. This may not be possible in multi-cylinder engines equipped with conventional mechanical cam phasing systems (and not having individual cylinder VVA capability) that are not able to implement individual, cylinder-specific valve lift profiles and timings.
Therefore, it is advantageous to have a control system for an internal combustion engine, including one intended to operate using a controlled auto-ignition process, which controls aspects of engine operation to accommodate changes in thermal characteristics of the combustion chamber surface which are due to combustion chamber deposits. Furthermore, there is a need for a practical way to determine magnitude of CCDs in the combustion chamber as a control input so that engine control systems can account for observed changes in combustion phasing and burn rate.